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373. PCK5/PCK6: Molecular Packing Analysis in Crystals
by Donald E. Williams, Department of Chemistry, University of Louisville, Louisville, Kentucky 40208 The crystal lattice energy is minimized by the least- squares, Newton-Raphson, or steepest-descent method as a function of the structural parameters.The simplified program, PCK5, uses the observed lattice constants and finds the optimum rotational and translational positions of the molecules in the cell. Quadratic nonbonded potentials are used with least squares minimization. The larger program, PCK6, includes the lattice constants as variables and allows intramolecular rotations about bonds. The nonbonded potentials are of the (exp-6-1) type, plus internal rotation potentials. Accelerated convergence of the lattice sums is used. Refinement proceeds via steepest descents, switching to Newton-Raphson as permitted. The program is dimensioned for 100 atoms in the asymmetric unit, including hydrogens (easily changed); less than 32 Kb word storage is required. _________ References: PCK5:D. E. Williams, "A Method of Calculating Molecular Crystal Structures," Acta Cryst., A25, 464 (1969). PCK6:D. E. Williams, "Molecular Packing Analysis," Acta Cryst., A28, 629 (1972). _________ FORTRAN IV Lines of Code: 4072 Recommended Citation: D. E. Williams, QCPE 11, 373 (1979). |